#include <bits/stdc++.h>
using namespace std;
struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define vv(type, c, m, n, i) vector<vector<type>> c(m, vector<type>(n, i))
#define rep(i,a,n) for(int i=(a), i##_len=(n); i<i##_len; ++i)
#define rrep(i,a,n) for(int i=(a), i##_len=(n); i>i##_len; --i)
#define each(x,y) for(auto &(x):(y))
#define var(type, ...)type __VA_ARGS__;Scan(__VA_ARGS__);
template<typename T> void Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void Scan(First& first, Rest&...rest) { cin >> first; Scan(rest...); }
#define vec(type, c, n) vector<type> c(n);for(auto& i:c) cin>>i;
#define len(x) ((int)(x).size())
#define mp make_pair
#define eb emplace_back
#define em emplace
#define pb pop_back
#define fi first
#define se second
#define get(a, i) get<i>(a)
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long long> vll;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef pair<int, int> pii;
#define mini min_element
#define maxi max_element
#define sum(a, b) accumulate(a, b)
#define chmax(a,b) if(a<b)a=b
#define chmin(a,b) if(a>b)a=b
#define ce(x, y) (x + y - 1) / y
template <class T> inline void print(T t){cout << t << "\n";}
template <class H, class... T> inline void print(H h, T... t){cout << h << " ";print(t...);}
int main() {
	var(int, n, k);
	vec(int, a, n);
	vi v(all(a));
	sort(all(v));
	int ans = 0;
	rep(i, 0, n - k + 1) {
		int m = n;
		while (m > 0) {
			for (int j = i ; j < m - k; j += k) {
				if (a[j] > a[j + k]) {
					swap(a[j], a[j + k]);
					++ans;
				}
			}
			m -= k;
		}
	}
	print(v == a ? ans : -1);
	return 0;
}